Field of the Invention
The present invention relates to magnetic brush development apparatus which provides improved development of electrostatic images formed on photoconductors, and especially those photoconductors that are moving at a relatively high rate of speed.
In electrography, it is common to form an electrostatic image on an insulating surface of an electrophotographic member in the form of a drum or web. Toner particles are applied to this image to develop it. In many commercial applications, the toner is either transferred in an image-wise configuration to another surface and then fixed or is fixed to the insulation surface itself. In processes in which the toner is transferred from the insulating surface prior to fixing, the insulating surface generally is reused.
Triboelectric developing systems are frequently used in the development of electrostatic images. In such systems, finely divided toner particles are held to the surface of much larger carrier particles by electrostatic charges created by triboelectrification, forming a mixture (herein called a developer). When the developer is brought into contact with an electrostatic image, the charge on the image attracts the triboelectrically charged toner and overcomes the attraction of the carrier for the toner thereby developing the image.
Among triboelectric developing apparatus, the most commonly used are cascade apparatus and magnetic brush apparatus. In cascade apparatus, gravity is used to roll developer across the image. Because cascade apparatus use gravity as their primary moving force, they generally are used in low-speed copiers. One reason for this is that in a high-speed copier, a cascade apparatus would require substantial space to effectively tone electrostatic images.
In magnetic brush apparatus which use a developer having carrier and toner particles, the carrier particles are ferromagnetic in nature. These ferromagnetic carrier particles are held to an applicator surface, for example, a nonmagnetic cylinder, in a bristle formation, by magnets located inside the cylinder. The bristles are brushed across a surface carrying an electrostatic image. The electrostatic attraction between the toner and the charged image overcomes the triboelectrically created attraction between toner and ferromagnetic particles and the image is developed. Areas of the image exerting less attractive force on the toner than is exerted by the carrier are cleaned of toner as they are brushed. Magnetic brush apparatus also may use a single component developer such as is disclosed in U.S. Pat. No. 3,816,840 issued June 11, 1974 to Kotz.
Dual magnetic brush apparatus such as shown in commonly assigned U.S. Pat. No. 3,543,720 to Drexler et al, and in U.S. Pat. No. 3,640,248 to Nielander include two magnetic brushes or rollers which rotate counter-current with respect to the movement of a web-type photoconductor. Developer is transported from the transport or downstream brush to the upstream or development brush. In this way, a large surface area of developer (sometimes referred to as a footprint) is presented to an electrostatic image and large quantities of toner are available for image development. The particular arrangement has performed quite satisfactorily; however, as copier speeds increase, it suffers from the flaming problem. Flaming is when the leading edge of a large solid area is not fully developed. Flaming is related to the velocity of moving photoconductors. It has been determined that as the photoconductor velocity increases, flaming increases. A discussion of flaming is set forth in some detail in commonly assigned U.S. Pat. No. 4,292,921 to Kroll et al. As is disclosed in the Kroll et al patent and also in Research Disclosure No. 16126 (September 1977), a counter-current brush preceeded by a co-current brush can reduce flaming. Unfortunately, when development of an electrostatic image on a high-speed photoconductor is needed, these brushes may not apply sufficient toner to such image to uniformly develop large solid areas.
In order to enhance development of electrostatic latent images, reduce flaming, and increase development of large solid images moving at high speed, many magnetic brush development apparatus include four or more brushes. See, for example, U.S. Pat. No. 3,911,864 where the magnetic brushes rotate in alternate directions, and U.S. Pat. Nos. 4,027,621, and 4,086,006, where all magnetic brushes rotate co-current.